This invention relates to the field of testing transistor and other active electronic devices and to the prevention of oscillating conditions during certain types of dynamic testing of high performance electronic devices.
In optimizing a new transistor for a particular use in an integrated circuit or a discrete component electronic application, it is frequently desirable to characterize and otherwise test the operation of the new transistor type in a testing environment which electrically simulates but is physically distinct from the operating environment expected for other transistors of the new type. Such testing often employs failure accelerating conditions which include elevated temperatures and other stresses along with periodic evaluation of transistor performance and mathematical extrapolations of the transistor's performance into a normal set of operating conditions. The use of mathematical models commonly referred to as Arrhenius equations for such extrapolations is for example now an accepted life test and reliability prediction tool in the semiconductor industry. Additional information concerning such predictions is to be found in the article authored by M. S. Ash and H. C. Gorton, I.E.E.E. Reliability, vol. 38, p. 485 (1989).
The need to extract an exemplary transistor device from its intended source and load and other environmental conditions for operation, even though desirable from the testing and reliability prediction viewpoint, is frequently found to be accompanied by technical difficulties. Especially, when the high gain, high frequency transistors that are now common in the electronic art, are extracted from a normal microwave environment and operated, perhaps in large numbers, in a high stress environment are these technical difficulties likely to be encountered. The gallium arsenide heterojunction bipolar microwave transistor is a specific case in point for these difficulties.
A particularly troublesome example of these technical difficulties is the pronounced tendency of these transistors when operated under normal bias conditions, to enter a mode of oscillation at some microwave or other high frequency. For example, the well known Miller effect capacitance in combination with the unusually large gain bandwidth product (gain bandwidths of 20 GHz being common for such devices) almost assures difficulty in achieving an oscillation-free multiple transistor device life test or characterization test. The presence of an oscillating mode of operation during such testing often influences the transistor's operation in a manner which jeopardizes or confuses the desired life test data. In order to effectively measure the characteristics of transistors on life test or transistors under characterization testing, it has therefore become common practice to accomplish tests of these types while the transistor is operating in a purely DC operating mode that is by brute force, free of the oscillating conditions. In this practice the life test environment is arranged to enforce-especially by loading, this pure DC and oscillation free operation.
The present invention is believed to provide major assistance in achieving the desired test operating conditions for a plurality of tested transistor devices and to accomplish this improvement in a manner which is low in cost, physically convenient, and especially compatible with the high performance characteristics of present day transistor devices. More precisely, the present invention provides a tested transistor loading and communicating arrangement which is especially desirable for use with high temperature transistors including gallium arsenide and other transistors whose useful characteristics may extend well into the microwave range of operating frequencies.
Although the use of energy feeding and signal loading devices during transistor testing has been known in the electronic art, and has included such devices as commercially fabricated bias-Tee networks which may be located external to the high temperature testing environment of the transistors under test, such devices have proven to be impractical from the environment requirement, financial cost, physical size and electrical performance viewpoints. The presence of low temperature plastic components in known commercial bias-T circuits and their extremely large physical size with respect to transistors of the microwave type has made their use ineffective in many test environments. The present invention is believed to offer a significant improvement over these previously employed arrangements.